High temperature sealing material

ABSTRACT

Glass composition for use as sealing material in fuel cells, comprising a glass matrix with main components consisting of SiO 2 , Al 2 O 3 , and one or more compounds from group I metal oxides and/or group II metal oxides, and a filler material evenly dispersed in the matrix, wherein the filler material consists of particles of one or more refractive compounds from the group: MgO—MgAl 2 O 4 , stabilized zirconia, rare earth oxides, (Mg,Ca)SiO 3 , Mg 2 SiO 4 , MgSiO 3 , CaSiO 3 , CaZrO 3 , ThO 2 , TiO 2 , M II AlSi 2 O 8 , where M II =Ca, Sr or Ba.

BACKGROUND OF THE INVENTION

[0001] The invention concerns a glass composition for use as sealingmaterial in fuel cells! preferably in the solid oxide fuel cells (SOFC)of the stacked planar type. Typically, such fuel cells are composed ofY-stabilized ZrO₂, (YSZ) electrolyte with electrodes and contact layersto the electron conducting plate Interconnect (IC), which makes theseries connection between the cells. Gas tight sealings are vitallyimportant for the performance, durability and safety operation of thefuel cells including the manifold and heat exchanger.

[0002] The difficulties in providing a suitable sealing material arenumerous:

[0003] The sealing material should be able to adhere to the fuel cellcomponents at a heat treatment not higher than 1300° C. which is themaximum temperature heat treatment of a fuel cell stack, and beresilient in order to take up deformations, e.g. due to TEC differencesbetween the fuel cell components, and at the same being able towithstand a certain overpressure at the operation temperature whichrequire a viscosity of more than 10⁵dPas·s. The thermal expansioncoefficient (TEC) should be in the range 9-13·10⁻⁶K⁻¹ in order not toinitiate cracks in the fuel cell components. Furthermore, the sealingmaterial has to be stable over a time span of say 40.000 h withoutdeteriorating crystallization or reactions with the other materials aswell as with the ambient gasses, atmosphere containing steam, methane,hydrogen, carbonmonoxide and carbondioxide or nitrogen and oxygen. Glassor glass ceramic seals may fulfil the requirements established above andaccording to literature quite a range of potential glasses have beenreported: TABLE 1 SOFC SEALING MATERIALS Alkaline oxide silicate glassesNa₂O—CaO—SiO₂ Li₂O—ZnO—Al₂O₃—SiO₂ and MgO—ZnO—SiO₂ Alkali-Al₂O₃—SiO₂Alkali-B₂O₃—SiO₂ Na₂O—SiO₂ Li₂O—SiO₂ Mica Glass Ceramics Commerciallyavailable mica glass-ceramic Alkaline-Earth OxideBorosilicate/Silicaborate Glasses Alkaline-Earth-B₂O₃—SiO₂SrO—La₂O₃—Al₂O₃—B₂O₃—SiO₂/ SiO₂—B₂O₃ BaO—As₂O₃—Al₂O₃—B₂O₃—SiO₂Alkaline-Earth Alumina Silicates SiO₂ based glass-ceramicsMgO—Al₂O₃—SiO₂ CaO—Al₂O₃—SiO₂

[0004] According to Ley et al. (1996), each of these glass types have adrawback: the alkalis and alkali silicates and borates will react withthe fuel cell components. The alkali borate glasses have too low TEC andsoda-lime glasses too low viscosity.

[0005] In contrast several glass compositions within theSrO—La₂O₃—Al₂O₃—B₂O₃—SiO₂ system should be suitable (K. L. Ley, M.Krumpelt, R. Kumar, J. H. Meiser & I. Bloom, 1996, J. Mater. Res., Vol.11, No 6, pages 1489-1493).

[0006] The present invention is in contrast to the conclusion of theauthors above based on highly viscous polymerizedalkali-alumina-silicate glass seals, which are reluctant to crystallizeat elevated temperature. An example of a highly polymerized glass ispure SiO₂ glass, which has a polymerized 3D network (as the crystallinephase, quartz) based on Sio₄ ⁴⁻ tetrahedra, where each oxygen ionconnects two Si ions (B. E. Warren & Biscoe, 1938, J. Am. Ceram. Soc.21, page 29). By addition of group I, II and III metal oxides thisnetwork is broken and the softening point, the viscosity and the meltingpoint decreases significantly. It is possible to retain a polymerizedstructure of the melt with a high viscosity by substituting SiO₂ withNaAlO₂ (D. C. Boyd & D. A. Thompson, in Ullmann, Vol. 11, page 815).Accordingly, a NaAlSi₃O₈ melt has a high viscosity of 10^(8.5) dPas·s at1120° C. (H. Rawson, 1967, Academic Press, London and New York, page89). This melt is assumed to have a 3D network (Si_(1-x), Al_(x))O₄^(4−x−) network structure, where xNa⁺ compensate the extra negativecharge, similar to the 3D network in the mineral albite with the samecomposition. Crystallization from such a highly viscous melt held nearly100° C. below the melting point may take years due to the high viscosity(H. Rawson, 1967). By addition or subtraction of NaAlSiO₄, SiO₂, it ispossible to reach two eutectic melting temperatures at 10620 and 10680at compositions: NaAlSiO₄: SiO₂, 37.0:63.0 wt % and 65.0:35 wt %,respectively (J. F. Schairer, J. Geol. 58, No 5, 514, 1950). For thesystem: KAlSiO₂, SiO₂, an eutectic point of 990+20° C. may be obtainedat a composition of KAlSiO₄: SiO₂ equal to 32.8:67.2 wt % (J. F.Schairer, N. L. Bowen, Bull Soc, Geol. Finland, 20.74 (1947).

[0007] The TEC of a NaAlSi₃O₈ glass is 7.5·10⁻⁶K⁻¹, which is lower thanthe SOFC components 10.0-13·10⁻⁶. The TEC of the albite glass can beincreased slightly by addition of NaAlO₂, whereas a value of10.4·10⁻⁶K⁻¹ may be obtained by addition of Na₂O giving a cationcomposition of Na_(3.33)Al_(1.67)Si₅ (O. V. Mazurin, M. V. Streltsina &T. P. Shvaikoshvaikoskaya, Handbook of glass data, part C, page 371,from K. Hunold & R. Brûckner, 1980a, Glastech. Ber. 53, 6, pages149-161). Higher values up to more than 12×10⁻⁶K⁻¹ can be obtained byfurther addition of Na₂O according to these authors.

[0008] An example of a NaAlSi₃O₈+Na₂O TEC matched glass for sealingyttria stabilized zirconia is shown in FIG. 1. Addition of K₂O will havean even higher effect on the TEC.

[0009] Addition of Na₂O and K₂O alone will decrease the viscosity andthe T_(glass) and T_(softening) as illustrated in Table 2 for Na₂O,which will be necessary for operation temperatures below 1000° C. TABLE2 T_(g) (° C.) T_(s) (° C.) 11.8 Na₂O − 19.4Al₂O₃ − 68.7SiO₂ 786 91017.1 Na₂O − 14.9Al₂O₃ − 68.0SiO₂ 515 607 11.8 Na₂O − 19.4Al₂O₃ −68.7SiO₂ + YSZ 814 929

[0010] Alkali-addition, however, will cause an increased reaction ratewith the other fuel cell components and an evaporation of sodium andpotassium, so that this solution is best suited for low operationtemperatures. Small amounts of BO₃ addition can also be used to decreasethe melt temperature and viscosity. An alternative to the addition ofalkalies in order to increase the TEC is to use fillers with a high TECand (Y. Harufuji 1992: Japanese Patent No 480,077 A2) and (Y. Harufuji1994, Japanese Patent No 623,784 A2) thus Harufuji mentions differentfibres of carbon, boron, SiC, polytitanocarbosilane, ZrO₂ and Al₂O₃ andpowders of Al₂O₃, ZrO₂, SiO₂, MgO, Y₂O₃ and CaO and Al, Ag, Au and Pt.To this list we can add stabilized ZrO₂, TiO₂, MgO—MgAl₂O₄ composites,(Mg,Ca)SiO₃, Mg₂SiO₄, MgSiO₃, CaSiO₃, CaZrO₃ and M^(II)AlSi₂O₈, whereM^(II)=Ca, Sr and/or Ba (rare earth oxides, e.g. CeO₂, Eu₂O₃ and ThO₂)(Li₂Si₂O₅ may be used at temperatures below 1000° C.).

[0011] Other alkalisilicates may be used as fillers for low temperatureoperation. A combination of alkali and filler addition can be used toobtain optimal TEC, viscosity and the softening point Ts. Also additionof small amounts (<wt %) B₂O₃ instead of or together with Na₂O combinedwith addition of high TEC fillers mentioned above is a possibility. Thefiller addition will reduce the exposed surface of the glass and thusthe evaporation of the more volatile constituents of the glass.

[0012] Deteriorating reactions may involve:

[0013] (1) SiO evaporation may occur under reducing condition on theanode side condensation may take place in other areas of the fuel cellsystem. Apparently this process is slow.

[0014] (2) Volatile sodium and potassium may react with the other fuelcell materials, e.g. the chromite of the interconnection plate. Theevaporation is strongly influenced by the sodium surplus of the glass.For this reason the sealing glasses with alkali/Al-ratios above 1 shouldonly be used in fuel cells with low operation temperatures.

SUMMARY OF THE INVENTION

[0015] According to the invention there is provided a glass compositioncomprising a glass matrix with main components consisting of SiO₂, Al₂O₃and one or more compounds from group I metal oxides, and a fillermaterial evenly dispersed in the matrix, wherein the filler materialconsists of particles of one or more refractive compounds from thegroup: (Al₂O₃, MgO,) rare earth oxides, MgO—MgAl₂O₄ composites,stabilized zirconia, (MgCa)SiO₃, Mg₂SiO₄, MgSiO₃ CaSiO₃, CaZrO₃, ThO₂,TiO₂ and the M^(II)AlSi₂O₈, where M^(II)=Ca, Sr and/or Ba. For lowtemperature application alkalisilicate fillers may be used.

[0016] The filler material is added to the sealing glass in order toadjust the thermal expansion coefficient, so that it matches the TEC ofthe other parts of the fuel cell in addition the stability of the glassmay be improved and the viscosity increased.

[0017] Preferred embodiments of the composition contain Na₂O or K₂O orboth in the amounts as indicated in claims 2 and 3 in order to reach anoptimum TEC, while at the same time avoiding too much alkali metal thatmay react with the other materials in the cell stack.

[0018] One or more compounds of group II metal oxide may be componentsof the glass matrix as indicated in claim 4.

[0019] Known glass compositions with main components comprising SiO₂,Al₂O₃, and one or more compounds from group I or group II metal oxidesare advantageous for sealing fuel cells with gas separators ofLa—Sr/Ca/Mg—Cr/V—O inter-connections of ceramic material or a metalalloy, e.g. Cr—Fe—Y₂O₃ material and with an operating temperature above600° C., cf. claims 6 and 7. In particular, compositions that within theglass have evenly dispersed a refractive filler material consisting ofparticles of one or more compounds from the group: MgO, MgO—MgAl₂O₄composites, stabilized zirconia rare earths (especially Eu₂O₃ and CeO₂),ThO₂, TiO₂, (MgCa)SiO₃, Mg₂SiO₄, MgSiO₃, CaSiO₃, CaZrO₃, M^(II)AlSi₂O₈,M^(II)=Ca, Sr/or Ba and ThO₂ TiO₂.

[0020] In a preferred embodiment of the invention commercially availablefeldspar or nepheline syenite starting materials may be used for makingthe basic glass material.

[0021] The starting material is melted at about 1550° C. for one hour inan alumina or platinum crucible. The melted material is then quenched inwater, crushed and ground to glass powder with a particle size of lessthan 90 μm. The glass powder having a TEC of about 75×10⁻⁷/K is thenmixed with a filler, e.g. MgO (TEC 130×10⁻⁷/K) with a grain size of lessthan 10-40 μm in ratio of 2:3.5 (vol.) in order to obtain a TEC of110×10⁻⁷/K.

[0022] Glass sealing are produced by filling the mixed powder intographite forms followed by stamping and removal of excess of powder. Thepowder is then sintered in a furnace in N₂-atmosphere at 750° C. for 5hours and at 1300° C., for one hour. Glass seals for narrow gaps <1 mm(e.g. in the electrode area) are produced by tape-casting of glass andfiller mixtures.

EXAMPLE 1

[0023] A commercial available feldspar (SiO₂=68.4, Al₂O₃=19.1,Fe₂O₃=0.1, CaO=2.0, Na₂O=7.5=K₂O=2.8; melt point 1270° C.) is melted at1550° C. in one hour in a Al₂O₃ or Pt crucible. Subsequently, the meltis quenched in water from 1000° C., crushed and grounded to ≦90 μm in aporcelain mill half filled with porcelain balls. The glass powder thusobtained with a TEC of ˜75×10⁻⁷/K is mixed with the preferred filler ofcalcined (1700° C.) MgO with a TEC of 130×10⁻⁷/K and a grain size of10-40 μm in a ratio of glass MgO of 2:3.5 in order to obtain a TEC of110×10⁻⁷/K. Glass sealings in the shape of triangular rods are producedby filling the mixed powder into a graphite cast form with V-shapedgrooves, followed by stamping and removal of excess powder. Thecomposition is sintered in a furnace in N₂-atmosphere at 750° C. for 5hours and at 1300° C. for 2 hours. The produced triangular rods may bemachined to the wanted dimensions. If the viscosity is too high additionof small amounts (<5%) B₂O₃ and/or Na₂O may be an advantage.

EXAMPLE 2

[0024] As in Example 1 except that the commercial feldspar powder (<90μm) is mixed with the calcined (1700° C.) MgO powder and melted ingraphite cast forms above 1300° C. for 5 hours.

EXAMPLE 3

[0025] Glass is produced from dried powder of: SiO₂ 1442.08 g (<1 μm)Al₂O₃ 407.84 g (<1 μm) Na₂CO₃ 423.9 g (>1 μm)

[0026] The powder is mixed in a 5 liter PE bottle with 20-30 mmporcelain balls for 24 hours. The powder is poured into Al₂O₃ cruciblesand heated to 1550° C. for 2 hours and subsequently quenched from 600°C. in H₂O. The quenched materials thus obtained is crushed to <9 mmgrains and grounded in a porcelain mill for 17 hours with 20-30 mm sizedballs to glass powder with a particle size of less than 100 μm.

[0027] The glass powder is then mixed with 50 wt %, ZrO₂ stabilized with8 mole % Y₂O₃ spherical 40-60 μm powder, calcined at 1700° C. and pouredinto a graphite cast form, as described in Example 1, and then sinteredat 750° C. for 5 hours and at 1125° C. for one hour followed by ovencooling. At FIG. 1 the expansion curve is given for this mixture.

EXAMPLE 4

[0028] Glass, produced as in Example 1 or 2, and a filler material (e.g.MgO or yttria stabilized zirconia) are mixed by ball milling for 2 hourswith a dispersing agent, polyvinylpyrrolidon dissolved inmethylethylketon and ethanol. Subsequently, a binder mixture ofpolyvinylbutyral, dibutylphtalat, polyethylenglycol, Additol (Hoechst)(dissolved in methylethylketon and ethanol) is added and the ballmilling is continued for 24 hours. The so-produced slurry is casted ontoa moving substrate and allowed to dry for 24 hours before removal.

[0029] The green tapes are cut into the desired dimensions and areapplied to the sealing area in the green state. The sealing is achievedby a subsequent heat treatment up to 1200° C. using a temperature rampbelow 50° C./h in the range up to 500° C.

EXAMPLE 5

[0030] A composition according to Beall (1986) (J. H. Simmons, D. R.Uhlmann, G. H. Beall (eds.) in 1982: Advances in Ceramics, Vol. 4, page291, Am. Ceram. Soc.) of: SiO₂ 38.9 Al₂O₃ 13.7 MgO 11.0 SrO 10.1 BaO14.0 MgF₂ 12.6

[0031] is produced by mixing the oxide components and MgF₂ in a highintensity vibration mill with Al₂O₃ balls. The composition is thenmelted at 1550° C. for one hour. The thus produced glass is milky with aTEC of 110×10⁻⁴/K according to Beall (1986). The glass is rather viscousand did not adhere well to the cell stack material. The adherence tothese materials can be improved by addition of small amounts (up to 5%)of B₂O₃ and/or alkaline metals.

1. A glass composition for use as sealing material in fuel cells,characterized in comprising a glass matrix with main componentsconsisting of SiO₂, Al₂O₃, and one or more compounds from group I metaloxides and/or group II metal oxides, and a filler material evenlydispersed in the matrix, wherein the filler material consists ofparticles of one or more refractive compounds from the group:MgO—MgAl₂O₄, stabilized zirconia, rare earth oxides, (Mg, Ca)SiO₃,Mg₂SiO₄, MgSiO₃, CaSiO₃, CaZrO₃, ThO₂, TiO₂, M^(II)AlSi₂O₈, whereM^(II)=Ca, Sr or Ba.
 2. A glass composition according to claim 1,characterized in that the glass matrix contains Al₂O₃ and Na₂O, wherethe stoichiometric molar ratio Al₂O₃: Na₂O is in the range of 0.1-1.3.3. A glass composition according to claim 1, characterized in that theglass matrix contains Al₂O₃ and K₂O, where the stoichiometric molarratio Al₂O₃: K₂O is in the range of 0.1 to 1.3.
 4. A glass compositionaccording to claim 1 including fluorine atoms as partial crystallizerforming a glass ceramic.
 5. A glass composition according to anyone ofthe preceding claims comprising additionally 0.1-10% of B₂O₃.
 6. Use ofa glass composition with main components comprising SiO₂, Al₂O₃, and oneor more compounds from group I metal oxides for sealing fuel cells at anoperating temperatures up to 1000° C.
 7. Use of a glass composition withmain components comprising SiO₂, Al₂O₃, and one or more compounds fromgroup II metal oxides for sealing fuel cells at operating temperaturesup to 1000° C.
 8. Use of a glass composition according to claim 5 or 6,characterized in that within the glass there is evenly dispersed afiller material consisting of particles of one or more compounds fromthe group: MgO—MgAl₂O₄, stabilized zirconia, rare earth oxides,(Mg,Ca)SiO₃, Mg₂SiO₄, MgSiO₃, CaSiO₃, CaZrO₃, ThO₂, TiO₂ andM^(II)AlSi₂O₈, where M^(II)=Ca²⁺, Sr²⁺ and/or Ba²⁺, Li₂Si₂O₅ may be usedas filler at operation temperatures up to 1000° C. and otheralkalisilicates at lower temperatures.